Citric Acid cycle or Tricarboxylic Acid cycle or Krebs Cycle
Overview and brief history
In Cytosol
•Pyruvate Dehydrogenase Complex (PDC) and its control
•Reactions of TCA cycle or CAC
•Amphibolic nature of TCA cycle
•Regulation of TCA cycle
In Mitochondria
•Reactions of Glycolysis are localized in Cytosol, and do not require any
oxygen.
whereas pyruvate dehydrogenase and TCA cycle reactions take place in
mitochondria where oxygen is utilized to generate ATP by oxydative
phosphorylation.
Consumption of oxygen (respiration) depends on the rate of PDC and
TCA reactions.
Historical perspective:
1930: Elucidation of Glycolysis
Study of oxidation of glucose in muscle,
addition of Malonate inhibited the respiration
(i.e. O2 uptake).
Malonate is an inhibitor of Succinate oxidation
to Fumerate
1935: Szent-Gyorgyi: demonstrated that little
amounts (catalytic amounts) of succinate,
fumerate, malate or oxaloacetate acelerated the
rate of respiration.
He also showed the sequence of inter-conversion:
Succinate --- Fumerate --- malate ---oxaloacetate.
1936: Martius & Knoop: Found the following sequence of reaction:
Citrate to cis-aconitase to Isocitrate to a Ketogluterate to succinate
1937: Krebs: Enzymatic conversion of Pyruvate + Oxaloacetate to citrate and CO2
Discovered the cycle of these reactions and found it to be a major pathway for
pyruvate oxidation in muscle.
Reaction of pyruvate dehydrogenase complex (PDC)
Reactions of TCA cycle: 8 reactions:
Citrate synthase
Aconitase
Iso-citrate dehydrogenase
α ketoglutarate dehydrogenase
Succinyl-Coenzyme A synthetase
Succinate dehydrogenase
Fumerase
Malate dehydrogenase
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Pyruvate dehydrogenase Complex (PDC)
It is a multi-enzyme complex containing three enzymes associated
together non-covalently:
E-1 : Pyruvate dehydrogenase, uses Thiamine pyrophosphate as
cofactor bound to E1
E-2 : Dihydrolipoyl transacetylase, Lipoic acid bound, CoA as
substrate
E-3 : Dihydrolipoyl Dehydrogenase
FAD bound, NAD+ as substrate
Advantages of multienzyme complex:
1. Higher rate of reaction: Because product of one enzyme acts as a
substrate of other, and is available for the active site of next
enzyme without much diffusion.
2. Minimum side reaction.
3. Coordinated control.
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Thiamin (Vitamine B1) deficiency causes Beriberi:
Thiamine pyrophosphate (TPP) is an important cofactor of pyruvate
dehydrogenase complex, or PDC a critical enzyme in glucose metabolism.
Thiamine is neither synthesized nor stored in good amounts by most vertebrates.
It is required in the diets of most vertebrates. Thiamine deficiency ultimately
causes a fatal disease called Beriberi characterized by neurological disturbances,
paralysis, atrophy of limbs and cardiac failure. Note that brain exclusively uses
aerobic glucose catabolism for energy and PDC is very critical for aerobic
catabolism. Therefore thiamine deficiency causes severe neurological symptoms.
Arsenic Poisoning: Arsenic compounds such as arsenite (AsO3---) organic
arsenicals are poisonous because they covalently bind to sulfhydryl compounds
(SH- groups of proteins and cofactors). Dihydrolipoamide is a critical cofactor of
PDC, and it has two-SH groups, which are important for the PDC reaction. These
–SH groups are covalently inactivated by arsenic compounds as shown below;
OH
-O
As
HS
Unfortunately and ignorantly, a common nineteenth century tonic, the
Fowler’s solution contained 10 mg/ml arsenite. This tonic must have
been responsible for many deaths, including the death of the famous
evolution scientist Charlse Darwin.
S
-O
+
OH
Arsenic compounds in low doses are very toxic to microorganisms,
therefore these compounds were used for the treatment of syphilis and
other diseases in earlier days. Arsenicals were first antibiotics, but with a
terrible side effects as they are eventually very toxic to humans.
HS
As
+ 2H2O
S
R
R
Arsenic Compound poisoning: Inactivation of E-2 of PDC, and other proteins.
Reactions of Citric Acid Cycle
Organic Arsenical were used
as antibiotics for the treatment
of syphilis and
trypanosomiasis.
Micro-organisms are more
sensitive to organic arsenicals
than humans.
1. Citrate synthase: Formation of Citroyl CoA intermediate.
2. Binding of Oxaloacetate to the enzyme results in conformational change
which facilitates the binding of the next substrate, the acetyl Coenzyme A.
There is a further conformational change which leads to formation of
products. This mechanism of reaction is referred as induced fit model.
But these compounds had
severe side effects and Aspoisoning.
Fowler’s solution, the famous
19th century tonic contained
10mg/ml As. Charles Darwin
died of As poisoning by taking
this tonic.
Napoleon Bonaparte’s death
was also suspected to be due
to As poisoning.
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2. Aconitase: This enzyme catalyses the isomerization reaction by
removing and then adding back the water ( H and OH ) to cis-aconitate
in at different positions. Isocitrate is consumed rapidly by the next
step thus deriving the reaction in forward direction.
4. α-Ketoglutarate dehydrogenase: This is a complex of different
enzymatic activities similar to the pyruvate dyhdogenase complex. It
has the same mechanism of reaction with E1, E2 and E3 enzyme units.
NAD+ is an electron acceptor.
3. Isocitrate dehydrogenase: There are two isoforms of this enzyme, one
uses NAD+ and other uses NADP+ as electron acceptor.
5. Succinyl CoA synthatse: Sccinyl CoA, like Acetyl CoA has a
thioester bond with very negative free energy of hydrolysis. In this
reaction, the hydrolysis of the thioester bond leads to the formation
of phosphoester bond with inorganic phosphate. This phosphate is
transferred to Histidine residue of the enzyme and this high energy,
unstable phosphate is finally transferred to GDP resulting in the
generation of GTP.
6. Succinate Dehydrogenase: Oxidation of succinate to fumarate. This
is the only citric acid cycle enzyme that is tightly bound to the inner
mitochondrial membrane. It is an FAD dependent enzyme.
Malonate has similar structure to Succinate, and it competitively inhibits
SDH.
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7. Fumarase: Hydration of Fumarate to malate: It is a highly
stereospecific enzyme. Cis-Maleate (the cis form of fumarate is not
recognized by this enzyme.
8. L-Malate dehydrogenase: Oxidation of malate to oxaloacetate: It is an
NAD+dependent enzyme. Reaction is pulled in forward direction by the
next reaction (citrate synthase reaction) as the oxaloacetate is depleted
at a very fast rate.
Conservation of energy of oxidation in the CAC: The two carbon acetyl
group generated in PDC reaction enter the CAC, and two molecules of CO2 are
released in on cycle. Thus there is complete oxidation of two carbons during
one cycle. Although the two carbons which enter the cycle become the part of
oxaloacetate, and are released as CO2 only in the third round of the cycle. The
energy released due to this oxidation is conserved in the reduction of 3 NAD+,
1 FAD molecule and synthesis of one GTP molecule which is converted to ATP.
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Efficiency of Biochemical engine in Living Systems:
Oxidation of one glucose yields 2840 kJ/mole energy
Energy obtained by biological engine: 32ATP X 30.5 kJ/Mol = 976
kJ/mol
Thus 34% efficiency is obtained if calculations are done using
standard conditions. But if concentrations in the cellular condition are
taken in account, the efficiency is close to 65%.
Anaerobic bacteria use incomplete citric acid cycle for production of
biosynthetic precursors. They do not contain a-ketoglutarate
dehydrogenase.
The amphibolic nature of Citric acid cycle: This pathway is utilized for the
both catabolic reactions to generate energy as well as for anabolic reactions to
generate metabolic intermediates for biosynthesis.
If the CAC intermediate are used for synthetic reactions, they are replenished by
anaplerotic reactions in the cells (indicated by red colours).
Fig. 16.16 Glyoxalate cycle
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Regulation of CAC:
Rate controlling enzymes:
Citrate synthatase
Isocitrate dehydrogenase
α-keoglutaratedehydrogenase
Regulation of activity by:
Substrate availability
Product inhibition
Allosteric inhibition or activation by
other intermediates
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